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  • Tweaks Turn Microscope into Billion-Pixel Imager
Jul 2013
PASADENA, Calif., July 30, 2013 — A method that converts a relatively inexpensive conventional microscope into a billion-pixel imaging system could improve the efficiency of digital pathology and provide robust microscopes to medical clinics in developing countries.

The physical limitations of microscope objectives have hindered conventional microscopes from improvement over the past several years. Microscope makers have tackled these limitations by using ever more complicated stacks of lens elements in microscope objectives to mitigate optical aberrations. Even with these efforts, the physical limitations have forced researchers to decide between high resolution and a small field of view, or low resolution and a large field of view.

Now, engineers at the California Institute of Technology (Caltech) have devised a system that combines the field-of-view advantage of a 2× lens with the resolution advantage of a 20× lens. The final images produced using the new system — which costs only about $200 to implement — contain 100 times more information than those produced by conventional microscope platforms.

Changhuei Yang’s lab at Caltech has developed a new microscope setup (left) that converts a relatively inexpensive conventional microscope into a billion-pixel imaging system that significantly outperforms the best available standard microscope. A raw image taken with a 2× objective lens is shown (top right) along with the reconstructed image produced by the new microscope setup (bottom right). Courtesy of Guoan Zheng.

“We found a way to actually have the best of both worlds,” said Guoan Zheng, lead author and initiator of the microscopy approach developed in professor Changhuei Yang’s lab. “We used a computational approach to bypass the limitations of the optics. The optical performance of the objective lens is rendered almost irrelevant, as we can improve the resolution and correct for aberrations computationally.”

An advantage of the approach, Zheng says, is its hardware compatibility. “You only need to add an LED array to an existing microscope. No other hardware modification is needed. The rest of the job is done by the computer.”

The system acquires about 150 low-resolution images of a sample, each corresponding to one LED element in an array. In the various images, light coming from known different directions illuminates the sample. A novel computational approach, Fourier ptychographic microscopy (FPM), is then used to stitch together the low-res images to form a high-resolution and more complete picture of the sample’s entire light field.

“What this project has developed is a means of taking low-resolution images and managing to tease out both the intensity and the phase of the light field of the target sample,” said Yang, a professor of electrical engineering, bioengineering and medical engineering at Caltech. “Using that information, you can actually correct for optical aberration issues that otherwise confound your ability to resolve objects well.”

Artist’s rendering of Caltech’s new microscopy setup showing one element of an LED array illuminating a sample. Courtesy of Yan Liang and Guoan Zheng.

The resulting large field of view could be particularly useful for digital pathology applications, where the typical process of using a microscope to scan the entirety of a sample can take tens of minutes. Using FPM, a microscope does not need to scan over the various parts of a sample — the whole thing can be imaged all at once. And, because the system acquires a complete set of data about the light field, it can computationally correct errors — such as out-of-focus images — so samples do not need to be rescanned.

“It will take the same data and allow you to perform refocusing computationally,” Yang said.

The method also could be used in everything from hematology to wafer inspection to forensic photography, the investigators say. The strategy could even be extended to other imaging methodologies such as x-ray imaging and electron microscopy.

“In my view, what we’ve come up with is very exciting because it changes the way we tackle high-performance microscopy,” Yang said.

The imaging strategy appeared online in Nature Photonics (doi: 10.1038/nphoton.2013.187). 

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forensic photography
The application of ultraviolet, x-ray, infrared and conventional photography to law enforcement.
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